LIPS AND NASAL APERTURES IN FISHES 153 
The current of water which enters the incurrent nasal aperture 
is accordingly at first directed against the oral wall of the nasal 
capsule, then turned aborally and mesially internal to the nasal 
_ valve, in the direction of the long axis of the fenestra nasalis and 
hence in the direction also of the median raphe of the Schneiderian 
membrane, and then again turned orally in order to issue through 
the excurrent aperture. The current thus has a zig-zag course 
through the nasal capsule, entering it at the oral and lateral end 
of the fenestra nasalis and leaving it at its mesial and aboral end. 
The secondary upper lips of this fish lie, as in Chlamydo- 
selachus, oral to the nasal apertures, and, as also in that fish, they 
do not extend to the median line. 
In Heptanchus the two nasal apertures lie near the lateral 
edge of the ventral surface of the snout, the incurrent aperture 
approximately lateral to the excurrent one. In Chlamydoselachus 
the two apertures lie still farther laterally, the incurrent aperture 
lying dorsal to the lateral edge of the snout and the excurrent 
aperture ventral to that edge. The positions of these apertures 
in these two fishes might accordingly be considered to represent 
two stages in a migration of the apertures from the ventral to 
the dorsal surface of the snout, such as is found in the ontoge- 
netic development of the Teleostei (His, 92 b), but this is quite 
certainly not the case, for this change in position of the apertures 
in the Teleostel is apparently due wholly to an unrolling of the 
cranial flexure, and not to a migration of the apertures, while 
the change in position in the Selachii, such as it is, is due wholly 
to their actual migration. 
In both Heptanchus and Chlamydoselachus, the median raphe 
of the Schneiderian membrane crosses the line of the external 
nasal apertures almost at a right angle, running forward and 
slightly mesially, approximately parallel to the upper edge of 
the mouth. In both fishes also the processes a and a’ of the 
ala nasalis are fused, the process a forming the external edge of 
the combined: processes; and this combined process and the 
process 6 project into the nasal capsule and form, together with 
that part of the ala nasalis which bounds the lateral (dorsal in 
Chlamydoselachus) half of the incurrent aperture, a broad cylin- 
